1. Field of the Invention
The present invention relates to a hydrogen gas sensor, and more particularly, to a hydrogen gas sensor suitable for measuring the hydrogen concentration of a fuel gas used for fuel cells.
2. Description of the Related Art
In view of the issue of global-scale environmental deterioration, fuel cells, which are clean and efficient power sources, have recently become the subject of active studies. Among fuel cells, a polymer electrolyte fuel cell (PEFC) is expected to be suitable for vehicle use due to its advantages, including low operation temperature and high output density. In this case, a reformed gas obtained from methanol or the like is advantageously used as a fuel gas. Further, in order to improve efficiency and other parameters of performance, a gas sensor capable of directly measuring hydrogen concentration of the reformed gas is needed.
Since such a hydrogen gas sensor is used in a hydrogen-rich atmosphere, the operation temperature of the gas sensor must be low (about 100xc2x0 C. or less). Such a low-operation-temperature-type sensor is disclosed in Japanese Patent Publication (kokoku) No. 7-31153. In the sensor, a working electrode, a counter electrode, and a reference electrode are disposed on an insulating substrate, and the three electrodes are integrally covered with a gas-permeable, proton-conductive film; more specifically, xe2x80x9cNAFION(copyright)xe2x80x9d (trademark, product of Dupont), which is a type of fluororesin. NAFION(copyright) is a proton-conductive material capable of operating at low temperature and is used at portions of polymer electrolyte fuel cells.
The present Inventors found that when NAFION(copyright) is used as a proton-conductive layer as in the gas sensor disclosed in Japanese Patent Publication No. 7-31153, the sensor output varies depending on the H2O concentration partial pressure of a gas under measurement (hereinafter referred to as a measurement gas atmosphere), so that accurate measurement becomes difficult. Further, the present Inventors found that the above phenomena occurs because protons pass through NAFION(copyright) together with H2O molecules, and therefore, the proton conductivity varies with the H2O concentration of the measurement gas atmosphere. That is, when the proton-conductive layer is formed of NAFION(copyright), the sensor output depends on the H2O concentration of the measurement gas atmosphere, so that the sensor output decreases greatly, especially when the H2O concentration is low.
The present Inventors further found that although porous Pt electrodes (catalysts) are generally known to exhibit high activity at low temperature (porous Pt electrodes are used, for example, in fuel cells), when such a Pt electrode is exposed to an atmosphere having a high CO concentration, CO is adsorbed on the Pt electrode, or the Pt electrode is CO-poisoned, so that the sensor output is greatly decreased.
Since many fuel cells use pressurized fuel gas in order to improve power generation efficiency, sensors used in the fuel gas are required to have a small pressure dependency. However, in the sensor described in the above-mentioned Japanese Patent Publication No. 7-31153, a gas under measurement is diffused to the working electrode via the gas-permeable, proton-conductive film, so that the sensor exhibits a great pressure dependency, depending on the structure of the proton-conductive film itself, and therefore high measurement accuracy cannot be obtained.
It is therefore an object of the present invention to provide a hydrogen gas sensor capable of accurately measuring hydrogen concentration in the presence of a variety of interfering gasses.
In the hydrogen gas sensor of the present invention, the rate of conduction of protons from a first electrode to a second electrode is rendered greater than the rate at which protons are derived from hydrogen introduced onto the first electrode via a diffusion-rate limiting portion.
That is, because the rate of conduction of protons from the first electrode to the second electrode is sufficiently greater than the rate at which protons are derived from hydrogen introduced from the measurement gas atmosphere onto the first electrode via the diffusion-rate limiting portion, the sensor can accurately measure hydrogen concentration without causing a great decrease in sensor output. That is so even when the measurement gas atmosphere has a low H2O concentration or a high CO concentration.
The present invention is applicable to both a hydrogen gas sensor not having a reference electrode and to a hydrogen gas sensor having a reference electrode. In the latter gas sensor, the voltage applied between the first and second electrodes can be variably controlled such that a constant voltage is produced between the first electrode and the reference electrode, or such that the hydrogen concentration on the first electrode becomes constant. Therefore, for any given hydrogen concentration an optimal voltage can be applied between the first and second electrodes, so that a more accurate measurement of hydrogen concentration can be obtained within a wide range of concentration.
The hydrogen gas sensor according to the present invention is advantageously used for measuring an atmosphere in which hydrogen H2O, and other components coexist, especially for measuring the hydrogen concentration of a fuel gas for polymer electrolyte fuel cells.
In a preferred mode of the present invention, the diffusion-rate limiting portion preferably has a relatively high gas-diffusion resistance, so as to render the proton-conducting performance excessive. In this case, the rate of conduction of protons through the proton-conductive layer becomes greater than the rate at which protons are derived from hydrogen introduced onto the first electrode. The gas-diffusion resistance of the diffusion-rate limiting portion is increased, for example, by increasing the length (thickness) of the diffusion-rate limiting portion in the gas diffusion direction or by decreasing the cross sectional area perpendicular to the gas diffusion direction (hereinafter referred to as a xe2x80x9cflow sectional areaxe2x80x9d). Alternatively, when the diffusion-rate limiting portion is formed of a porous material, the gas-diffusion resistance of the diffusion-rate limiting portion is increased by decreasing the porosity (pore diameter, apparent porosity, etc.) of the porous material.
The gas-diffusion resistance of the diffusion-rate limiting portion is preferably set as follows in order to render the rate of conduction of protons from the first electrode to the second electrode greater than the rate at which protons derived from hydrogen are introduced onto the first electrode via the diffusion-rate limiting portion.
(1) Proton Conduction Condition A
A proton-conducting rate under severe conditions is measured. That is, a current (a) flowing between the first and second electrodes is measured upon applying a sufficiently high voltage between the first and second electrodes in a state in which the gas-diffusion resistance of the diffusion-rate limiting portion is rendered sufficiently small (e.g., about 0.9 mA/mm2 or more of the first electrode (3), with current conversion, at H2=40%) in order to introduce a sufficiently large amount of hydrogen onto the first electrode, but under the severest conditions for proton conduction; e.g., conditions such that the measurement gas atmosphere has a very low H2O concentration (specifically, 10% or less at 80xc2x0 C.) or a very high CO concentration (specifically, 1000 ppm or greater). Although the above-described current (a) need not be a saturation current, the applied voltage (specifically, 50 mV or higher) is preferably equal to or higher than the voltage applied in the case of condition B described below.
(2) Proton Conduction Condition B
Next, a proton-conducting rate under favorable conditions is measured. That is, a saturation current (b) flowing between the first and second electrodes is measured upon application of a sufficiently high voltage between the first and second electrodes in a state in which the gas-diffusion resistance of the diffusion-rate limiting portion is rendered larger (e.g., less than about 0.9 mA/mm2 of the first electrode (3), with current conversion, at H2=40%) in order to sufficiently reduce the amount of hydrogen introduced onto the first electrode, but under favorable conditions for proton conduction, e.g., conditions such that the measurement gas atmosphere has a sufficiently high H2O concentration (specifically, 15% or greater, more preferably 20% or greater, at 80xc2x0 C.) or a sufficiently low CO concentration (specifically, 800 ppm or less). The sufficiently high voltage for producing a saturation current (b) is 300 mV or more at H2=40%, and varies according to the H2 concentration as shown in FIG. 2. In the case of H2=10%, it is about 100 mV or more.
(3) Setting of Gas-diffusion Resistance
When the gas-diffusion resistance of the diffusion-rate limiting portion is set to a sufficiently high value under condition B, current (a) greater than saturation current (b). Thus, the hydrogen gas sensor is configured such that proton-conducting rate (current value) under the severest conditions for proton conduction greater than proton-conducting rate under favorable conditions for proton conduction. In this hydrogen sensor, the proton-conducting rate is always greater than the proton-generation rate corresponding to the rate at which hydrogen is introduced onto the first electrode (or the largest proton-generation rate corresponding to the largest rate at which hydrogen is introduced onto the first electrode).
In yet another preferred mode of the present invention, current (c) flowing between the first and second electrodes is measured under severe conditions for proton conduction; current (d) flowing between the first and second electrodes is measured under favorable conditions for proton conduction; and the gas-diffusion resistance of the diffusion-rate limiting portion is set such that the ratio of current (d) to current (c) (=current (d)/current (c)) or its reciprocal (=current (c)/current (d)) approaches 1. As a result, the H2O -concentration dependency and CO-concentration dependency of the current flowing through the first and second electrodes decrease. Preferably, the gas-diffusion resistance of the diffusion-rate limiting portion and/or the area of the first or second electrode is properly set, or a predetermined polymer electrolyte solution is applied to the interface of the first or second electrode which is in contact with the proton-conductive layer, such that the ratio (saturation current flowing between the first and second electrodes at H2O=30%)/(saturation current flowing between the first and second electrodes at H2O=10%) falls within the range of 1 to 1.5, preferably 1 to 1.15, more preferably, 1 to 1.1, most preferably, 1 to 1.05. Further, preferably the gas-diffusion resistance of the diffusion-rate limiting portion and/or the area of the first or second electrode is appropriately set, or a polymer electrolyte solution is applied to the interface of the first or second electrode which is in contact with the proton-conductive layer, such that the ratio (saturation current flowing between the first and second electrodes at CO=1000 ppm)/(saturation current flowing between the first and second electrodes at CO=0 ppm) falls within the range of 0.9 to 1 (the reciprocal of the rate falls within the range of 1 to 1.1), more preferably, 0.95 to 1 (the reciprocal of the rate falls within the range of 1 to 1.05). Thus, a layer containing a polymer electrolyte is formed at the interface.
In a preferred mode of the present invention, the first and second electrodes are formed in an opposed manner to sandwich the proton-conductive layer. This structure reduces the resistance between the first and second electrodes to thereby improve the proton-conducting performance of the proton-conductive layer. However, when the gas-diffusion resistance of the diffusion-rate limiting portion is excessively high, the sensitivity of the hydrogen gas sensor is lowered. Therefore, the area of at least one of the first and second electrodes is preferably increased when the sensor must have a relatively high sensitivity. Further, the first and second electrodes may be formed on the same plane of the proton-conductive layer, if a sufficient degree of sensitivity can be achieved.
In a preferred mode of the present invention, a solution containing a polymer electrolyte identical to that of the proton-conductive layer is applied to the side of each electrode in contact with the proton-conductive layer (the interface between each electrode and the proton-conductive layer). This increases the contact area between the proton-conductive layer and catalytic components carried on the electrode, so that the proton-conducting performance is further increased. Further, the proton-conducting performance may be enhanced by decreasing the thickness of the proton-conductive layer.
In a preferred mode of the present invention, the proton-conductive layer is a polymer electrolytic proton-conductive layer which sufficiently operates at a relatively low temperature, for example, at temperatures not greater than 150xc2x0 C., preferably, at temperatures not greater than 130xc2x0 C., more preferably, at around 80xc2x0 C.; e.g., a proton-conductive layer formed of a resin-based solid polymer electrolyte.
In a preferred mode of the present invention, a proton-conductive layer is formed of one or more types of fluororesins, more preferably of xe2x80x9cNAFION(copyright)xe2x80x9d (trademark, product of Dupont).
In a preferred mode of the present invention, each electrode is a porous electrode formed of carbon or another suitable material, and carries a catalyst such as Pt on the side of the electrode in contact with the proton-conductive layer.
In the preferred mode of the present invention, the proton-conductive layer, the respective electrodes, and the diffusion-rate limiting portion are supported on a support to thereby constitute an integrated hydrogen gas sensor. The support is formed of an inorganic insulating material such as an alumina ceramic or an organic insulating material such as resin. Further, the diffusion-rate limiting portion is preferably formed of a porous alumina ceramic or a like material having gas permeability, or alternatively may be formed of small holes each having a small flow sectional area, such as one or more through-holes each having a very small opening diameter, which are formed at a portion of the support formed of a dense body. Such fine through-holes can be formed by use of, for example, laser machining or ultrasonic machining. In the case of laser machining, the opening diameter may be adjusted by controlling the irradiation diameter, output power, time, etc., of a laser beam. The average pore diameter of the porous material and the opening diameter of the through-holes are preferably not less than 1 xcexcm. In this case, since gas diffusion proceeds outside the region of Knudsen diffusion, the pressure dependency of the sensor can be lowered.
The hydrogen gas sensor of the present invention can be fabricated by physically sandwiching the proton-conductive layer and the respective electrodes between two supports such that the respective electrodes contact the proton-conductive layer. Alternatively, the respective electrodes may be bonded to the proton-conductive layer by hot pressing.
In the preferred mode of the present invention, a hydrogen gas sensor not having a reference electrode has a support for supporting the proton-conductive layer, the first electrode, the second electrode and the diffusion-rate limiting portion, and a hydrogen gas sensor provided with a reference electrode has a support for supporting the proton-conductive layer, the first electrode, the second electrode, the reference electrode and the diffusion-rate limiting portion.